Electron beam lithography systems have been used for fabrication of LSI, VLSI and ELSI (extra large scale integration) circuits.
An electron beam lithography system has an electron gun for producing an electron beam. The beam is focused onto a material to be patterned such that the beam hits the desired position on the material. Thus, an IC pattern is drawn on the material. Electron beam lithography systems of this kind are classified into two major classes: spot beam electron beam lithography system, where an electron beam focused to a spot scans a desired region on the material to draw a desired pattern; and shaped electron beam lithography system, where an apertured plate having an aperture of a given shape or given shapes is placed in electron optics and an image of the aperture is focused at a desired location on the material, thus drawing a desired pattern. Electron beam lithography systems of the shaped beam or area type are further classified into a fixed shaped electron beam lithography system equipped with a single apertured plate and a variable shaped electron beam lithography system having a deflector between plural apertured plates.
In the variable shaped electron beam lithography system, the electron beam passed through the upper aperture is appropriately deflected by the deflector over the lower apertured plate so that the beam having a desired cross-sectional shape passes through the lower aperture. The variable shaped electron beam lithography system is disclosed in U.S. Pat. No. 4,117,340. An electron beam lithography system equipped with both fixed and variable shaped beam systems has been proposed recently.
In the spot beam electron beam lithography system, images of the electron emission portions are focused to a spot on the material to be patterned. On the other hand, in the shaped electron beam lithography system, an aperture image conforming to the size of an aperture is focused onto a material to be patterned. Therefore, the shaped electron beam lithography system is capable of drawing a pattern with an electron beam current greater than that of the beam used by the spot beam electron beam lithography system and hence the former system provides a higher writing speed than the latter system. Consequently, the former system can obtain a higher throughput.
Normally, an electron beam lithography system uses a field emission emitter or heated lanthanum hexaboride (LaB.sub.6) as an electron gun cathode. In the electron gun equipped with the field emission cathode, the electron-generating portion has a quite high brightness B (about 10.sup.8 A/cm.sup.2.sterad.) and the spread of initial velocities .DELTA.v is narrow. Also, the spread of current densities is narrow. So, the current densities on the material are high. On the other hand, in the electron gun equipped with the heated lanthanum hexaboride, the electron-generating portion has a low brightness B (about 10.sup.6 A/cm.sup.2.sterad.) and the spread of the initial velocities .DELTA.v is wide. So, current densities on the material are low.
Let .alpha. be one-half of the incident angle of the electron beam to the material to be patterned. Let B be the brightness of the electron-generating portion of the electron gun. The current density of the electron beam impinging on the material to be patterned is given by EQU .rho.=B.pi..alpha..sup.2 ( 1)
Let V be the accelerating voltage applied to the electron beam by the electron beam lithography system. The resolution S of the image on the material to be patterned is in proportion to .DELTA.v/V.
Where the field emission electron gun is used in the above-described spot beam electron beam lithography system, images of electron emission portions can be focused onto the material with sufficiently high current density (for example, 1,000 A/cm.sup.2) and with a high resolution S (on the order of 0.01 .mu.m), because of high brightness and narrow velocity distribution. However, the diameter of the image focused onto the material is quite small, approximately 0.05 .mu.m, for example, because of the very small area of electron emission portions. This makes it impossible to increase the writing speed. For this reason, a high throughput cannot be expected. If the beam diameter is increased by defocusing the image formed on the material, the resolution of the image and the current density will deteriorate. A spot beam electron beam lithography system using an electron gun consisting of a field emission electron gun is disclosed by H. Nakazawa, H. Takemura, M. Isobe, Y. Nakagawa, M. Hassel Shearer and W. Thompson "A thermally assisted field emission electron beam exposure system" in J. Vac. Technol. B6(6), November/December 1988, pp. 2019-2022.
Where an electron gun equipped with a LaB.sub.6 cathode is used in the aforementioned shaped electron beam lithography system, an aperture image having a large size (for example, about 5 .mu.m.times.5 .mu.m) and exhibiting uniform current density distribution over the whole image can be focused onto the material. However, the brightness B of the electron-generating portion is low and so it is impossible to focus an aperture image of sufficiently large current density onto the material. Consequently, a very high throughput cannot be expected. Furthermore, the resolution of the aperture image is limited to about 0.1 .mu.m.
When a field emission gun is employed in the aforementioned shaped electron beam lithography system, it is possible to focus an aperture image having a large size (for example, about 5 .mu.m.times.5 .mu.m) and a high resolution S (about 0.01 .mu.m) can be focused onto the material. But it is impossible to focus a large aperture image with a sufficiently high current density. Hence, a high throughput cannot be expected.